Imagine that it’s your 30th birthday and after years of your parents asking about when you’re finally going to get married, they take it upon themselves to set you up with someone: your cousin. In the United States, most of us laugh at the absurdity of marrying our cousin, but in many Middle Eastern, Sub-Saharan African, and South Asian cultures, this practice is highly prevalent. Consanguineous marriage (CM) is the practice of marrying a blood-related individual who is a second cousin or closer; however, the large majority of cousin marriages occur between first cousins, especially between a man and his father’s brother’s daughter (FBD) . This type of marriage persists because it ensures that property and financial assets stay within the family, offers the security of marrying daughters to trusted spouses, and strengthens the paternal dominance in a lineage .
Seeing that the social consequences are generally constructive, what are the genetic consequences of FBD marriages? Examining the average shared DNA between relatives, researchers have found that we share about 50% of our genetic material with our siblings and parents, 25% with our aunts/uncles, and about 12.5% with our first cousins which is a significant, yet reconcilable amount of DNA to share with a spouse . A singular and isolated union between cousins slightly inflates the probability of a harmful recessive trait being expressed in the next generation. Fortunately, one instance of CM can be easily reversed by introducing new genetic material and re-establishing heterozygosity in the next generation . However, more significant issues arise as cousins continue to marry cousins within a family, each instance shrinking the available gene pool for the next generations. This creates a significant lack in genetic diversity which has two main effects — decreased resilience to disease and environmental changes, and the propagation and expression of dangerous alleles in a population.
In a study published by the Journal of Molecular Genetics and Genomic Medicine, researchers conducted an experiment at Hamad Medical Corporation, a tertiary center for the diagnosis and management for certain genetic disorders, on 599 Qatari families with certain indicators of genetic and nongenetic anomalies. Of this group, consanguineous marriages were observed in 397 (66.2%). The study found that “consanguineous marriages had significantly higher risk of Autosomal Recessive disorder compared to nonconsanguineous marriages'' with about 50% of consanguineous marriages having autosomal recessive conditions compared to 20% of nonconsanguineous marriages . Interestingly, the researchers found that the expression of recessive alleles was mainly observed in the autosomal chromosomes, meaning that, at least in this population, recessive sex-linked disorders are not propagated through cousin marriages.
Another study conducted on the risks of CM in Bangladesh found that the mortality rate of children under 5 years of age was significantly higher in CM families compared to non-CM families. Moreover, there was an observed increase in both child mortality rate and miscarriage with increased levels of inbreeding, or how closely related the parents are to each other. It was also discovered that CM is associated with the incidence of heart diseases, bronchial asthma, sickle cell anemia, and hearing defects. Despite the tragic effects that CM has had on the population, researchers noticed that “the general attitude and perception toward CM were rather indifferent, and very few people were concerned about its genetic burden” . This qualitative remark has two potential implications: those affected by the genetic consequences of CM have either not been made aware of the threat it poses or that they are aware, but do not value the greater health of their community above the preservation of a harmful tradition. Yet, both studies agree that consanguineous marriages are a large risk to the populations they’re practiced in, and that greater social and public health initiatives need to be taken to mitigate the prevalence of genetic diseases.
One particularly analytical perspective proposed the idea that CM has certain biological advantages in a population claiming that “the consequence of consanguineous marriages is an upsurge in the number of homozygous diseased individuals with fewer chances of mating and reduced chances of survival, therefore evolutionarily confining the transmission of disease alleles to future generations and encouraging its elimination from a population” . From a purely functional perspective, the idea of containing recessive diseases within a single community to promote its removal from an entire population seems like an effective course of action. However, even if the immense social and humanitarian issues that this idea presents are put aside, the isolation of a recessive allele is a temporary solution to a reliably imperfect DNA replication process that inevitably results in another mutated allele appearing within the population just as quickly as the first was eliminated.
While we have yet to discover a method for treating genetic conditions directly, there are measures we can take to reduce the harmful effects they have on public health, starting with the reduction of consanguineous marriages. This type of cultural change does not solely require widespread awareness campaigns and policy at the government level restricting CM; it needs time — time for people to shift their values towards their collective health and away from archaic traditions.
Waller, Donald M. , & Keller, Lukas F. (2020). Inbreeding and Inbreeding Depression. obo in Evolutionary Biology. doi: 10.1093/obo/9780199941728-0124